Automated manufacturing equipment relies increasingly on non-contact electronic scanners, such as sonar scanners or optical scanners, to determine the location and dimension of articles being processed by the equipment. The outputs from these scanners are typically fed to computer control systems which dictate the further processing of the articles based on their dimensions or positions.
A number of optical scanning systems have been used in the prior art with varying degrees of success. One popular system is known as shadow scanning and involves a linear array of dozens of light sources opposite a corresponding linear array of photodetectors. The object being scanned is positioned between the sources and photodetectors, causing various of the light beams to be interrupted. A computer monitoring the photodetectors notes which of the light beams are interrupted, thus indicating one dimension of the object. By advancing the object through the "light curtain" at a uniform rate and sampling the photodetector outputs at periodic intervals, additional data can be obtained about the configuration of the object in a second dimension.
Shadow scanners suffer from a number of drawbacks. One is that a scanner with dozens of discrete light sources and dozens of discrete photodetectors arranged over what may often be a ten or twenty foot area is inherently costly, complex and unreliable. If any of this multitude of components fail, operation of the system is impaired. Another drawback is that the shadow scanner provides no information about the configuration of the object in a dimension parallel to the light path.
In some applications a light reflection optical scanner such as a laser ranging system using triangulation techniques can advantageously be employed to characterize an object's configuration. In such a system a laser beam is projected obliquely against the object and a photoelectric camera is positioned to include in its field of view the point at which the laser intersects the object. If the object is relatively larger, the location at which the laser light intersects the object will appear to shift towards the side from which the laser light is emanating. Conversely, if the object is relatively smaller, the point of intersection will appear to shift in the opposite direction. By detecting in the camera output signal the position of the laser light on the object, the object's dimension relative to a reference location can be determined.
Laser ranging systems, too, suffer rom a number of drawbacks. Principal among these is the limited information they provide. In the foregoing example, the system provides information solely on the distance between the surface of the object and a reference location. Again, by moving the object relative to the laser scanner, cross-sections at different portions of the object can be obtained so as to determine a second dimension of the object. However, laser systems are unable to provide any information about the configuration of the object in the third dimension.
To characterize the shape of an object in three dimensions, an optical scanner system that includes two orthagonally positioned shadow scanners is sometimes employed. The object to be measured is first passed horizontally through a shadow scanner comprised of vertically spaced light sources and photodetectors. The object is next passed vertically through a second scanner comprised of horizontally spaced light sources and photodetectors. The data from these two scanners is then processed to roughly characterize the shape of two orthogonal sections through the length of the log. However, even with this elaborate system, the data provided is poor, as no information is obtained about object configuration between the orthagonally related sections.
Another type of optical scanning system for determining the shape of an object in three dimensions is shown in U.S. Pat. No. 4,301,373 and employs both laser ranging and light reflectance techniques. In this system, the object being scanned is illuminated in phases, first by a first neon strobe oriented obliquely towards the object, next by a second neon strobe oriented orthogonally to the first, and then by a laser beam. A camera records the reflectance of the object to each of these illuminations in separate scan data output signals which are then provided to a computer for processing. Since the reflectance of the object is a function of the illumination's angle of incidence, the computer can determine by a comparison of the two neon strobe data the inclination of the object's surface. The laser beam data provides information on the object's thickness.
While the foregoing system is useful in certain applications, its complexity and its inability to resolve certain surface inclinations renders it poorly suited for many applications.
Therefore, a need remains for a method and apparatus for characterizing the configuration of objects simply and reliably in two or three dimensions.